The invention relates in general to pulse generators, and more particularly to a pulse generator having an accurately selectable output frequency. Pulse generators are used as measuring signal sources for the generation of pulses with selectable repetition frequency and width, and with adjustable delay relative to a trigger signal.
Known pulse generators use an oscillator to produce an output signal having a frequency that is an integral fraction of the oscillator frequency. The integral ratio is selectable over a range of integer values to vary the frequency in discrete steps. In order to set the pulse generator to frequencies between these discrete values, these pulse generators use oscillators whose frequencies are adjustable in response to a control voltage. However, this type of analog frequency variation causes a frequency inaccuracy of several percent.
In accordance with the illustrated preferred embodiment, a pulse generator is presented which allows a highly precise continuous adjustment of the frequency, pulse width and pulse delay of the pulse generator output signal. The pulse generator uses a start-stop oscillator having a control input for stopping the oscillator in response to a stop pulse. During the duration of the stop pulse, the output of the oscillator remains at a logical "0" voltage. At the end of the stop pulse the oscillator is restarted. By adjusting the duration of the stop pulse, the delay time is adjusted to delay the leading edge of the first pulse following the stop pulse. The stop pulse occurs when a predetermined number of oscillator pulses have been counted. The output signal of the pulse generator is derived from the count values in such a way that at the occurrence of a predetermined count value an output pulse is started which continues until a second adjustable count value is reached. By the selection of the two count values the pulse delay and the pulse width of the output signal are selected.
The surveillance of the count values is accomplished by use of a pair of comparators, each of which compares the instantaneous count value with as associated stored nominal value. The output pulse is started when the count value equals the nominal value associated with the first comparator and is stopped when the count value equals the nominal value associated with the second comparator. Fine adjustment of the pulse delay and pulse width are obtained by adjustable delay lines connected to the outputs of the comparators for delaying the beginning and the end of the output pulse to the extent desired.
With the pulse generator according to the invention, the parameters of the output signal (i.e. pulse frequency, pulse width, pulse delay) are adjusted with high precision. The total frequency range may extend from the kHz range to several hundred MHz. Without large expenditure, an increase in frequency accuracy is obtained. Furthermore, the pulse generator has a high resolution, even at large values of period, delay and pulse width.